Field of the Invention
The invention relates generally to sensors used for recording fingerprints. In particular, the invention relates to an apparatus for protecting against ESD (Electrostatic Discharge) for such fingerprint sensors.
Every person has biological features with characteristics that are permanent from birth and that can be uniquely associated with the person to whom they belong. One of these biological features is fingerprints. Fingerprints have papillary lines with patterns including arcs, whirls and loops. The characteristics of these patterns are always unique. Hence, equipment intended for establishing the identity of a person frequently use the characteristics of a fingerprint to identify the person in question. Such equipment can be used in a wide variety of ways. Security systems based recognizing fingerprints are useful devices for protecting against misuse, particularly in the fields of e-commerce and electronic banking or at entrances to areas of buildings.
To record a fingerprint, capacitive fingerprint sensors are usually used. Such fingerprint sensors have a contact surface with an underlying matrix including a large number of individual sensor electrodes. The sensor electrodes have an AC voltage applied to them. As soon as a finger is placed in the vicinity of the matrix, electrostatic capacitances arise between the underside of the finger and the sensor electrodes to which the AC voltage is applied. The different dielectric constants of the papillary lines and the grooves in the fingerprints can then be used to distinguish between the papillary lines and the grooves. This allows the fingerprints to be represented in two dimensions. Such fingerprint sensors are known from Japanese Patent Application JP 8 305 832 A and from U.S. Pat. No. 6,055,324, which are herein incorporated by reference. In particular, these documents explain the function of the AC voltage applied to the sensor electrodes.
With fingerprint sensors, the intention is for the finger whose print needs to be recorded to directly touch the contact surface of the fingerprint sensor. When the finger touches the fingerprint sensor in this way, electrostatic charging can easily occur. The subsequent discharge or the resultant overvoltages can cause the operation of the fingerprint sensor to be impaired or can even cause the fingerprint sensor to be destroyed. To prevent the electrostatic discharge, it is not acceptable, in normal use of a fingerprint sensor, for the finger to have to be grounded in order to discharge it before contact with the fingerprint sensor. For this reason, protection against ESD is frequently ensured for the fingerprint sensor by situating a tungsten protective grating above the sensor electrode matrix. By way of example, this protective measure is described in Published German Patent application DE 199 01 384 A1, which corresponds to U.S. patent application Ser. No. US2002/0066942. U.S. patent application Ser. No. US2002/0066942 is hereby incorporated by reference. A drawback of a tungsten protective grating is that it reduces the sensitivity of the fingerprint sensor, because some of the electrical field lines running from the sensor electrodes to the contact surface are deflected by the tungsten protective grating. The sensitivity E of a fingerprint sensor is defined by the following equation:
                              E          =                                    C              useful                                      C              stray                                      ,                            (        1        )            where Cuseful is the useful capacitance between the sensor electrodes and the finger, and Cstray denotes the undesirable stray capacitance. The stray capacitance Cstray covers the capacitance between the sensor electrodes and the protective grating.
Sensitivity is a fundamental quality feature of a fingerprint sensor. Only if sensitivity is sufficient is it possible to record the diverse characteristics of a fingerprint.